A PC strand generally has a twisted structure of plural surrounding wires twisted around a core wire. The reason for using such a structure is to impart flexibility to the PC strand, and to form helical grooves with the twisted surrounding wires and thus provide a sufficient shear resistance for wires embedded in concrete. Accordingly, there is a need for a rustproof processing method for the PC strand that does not interfere with these characteristics. Currently, a number of rustproof processing methods for the PC strand are known.
One example of such known conventional techniques is “Rustproof film forming and processing method for PC strand” disdosed in Japanese Patent 2691113. In this technique, twisted portions of a PC strand are temporarily untwisted in sequence. The untwisted portions are maintained with a spread maintaining means, and the excess core wire is adjusted. A deposition coating of synthetic resin powder coating material is then formed on whole outer peripheral surfaces of the untwisted core wire and surrounding wires. The deposition coatings are then heat-fused to form films on the whole outer peripheral surfaces of the core wire and surrounding wires. The core wire and the surrounding wires are then twisted again after cooling the films.
The PC strand formed in this manner includes a film formed individually for the core wire and surrounding wires over the whole outer peripheral surfaces. Thus, the method does not interfere with the characteristics required as a PC strand, including flexibility, and the shear resistance of the strand embedded in concrete. Rustproofing is also sufficient. The rustproofing method of this publication is thus praised as the ultimate rustproofing method for a PC strand.
There are de facto standard film thicknesses in industry. Specifically, many research findings report that a thickness of 200 ±50 μm for powder epoxy resin coatings sufficiently satisfy corrosion performance and mechanical performance (impact resistance, flexural property, ease of concrete adhesion). Experiment results from The Federal Highway Administration (FHWA, US) report that the preferred film thickness is about 170 ±50 μm.
Another known conventional technique is “Method for forming a double film for PC strand” disclosed in Japanese Patent 3172486. In this technique, surrounding wires of a PC strand are temporarily untwisted from a core wire in sequence, and a rustproof film is formed on the whole outer peripheral surfaces of the core wire and the surrounding wires in the untwisted state. Then, the surrounding wires are twisted back while accumulating and absorbing the excess core wire resulting from the increased diameter. A protective film is then formed over the rustproof film. When a stably maintained maximum thickness of 250 μm or more is required for rustproof film subject to potential damage in some special structure, a double film is formed by forming a thick protective film on the outer peripheral surface of the PC strand formed by using the foregoing first conventional technique.
Another known conventional technique is “Method for forming rustproof film on PC strand” disclosed in Japanese Patent 3654889. In this technique, a PC strand formed after plating of its elemental wires is untwisted, and a resin film is formed on the whole outer peripheral surfaces of the core wire and the surrounding wires, which are then twisted again after cooling the resin film.